A tax on sugary drinks would reduce the number of obese adults in the UK by 180,000 and raise over £275m for the Treasury, according to doctors.

Researchers from Reading and Oxford universities say a standard 500ml bottle of some drinks contains as many as 14 teaspoons of sugar, or 210 calories; and they warn the drinks increase the risk of obesity, diabetes, cardiovascular disease and tooth decay.

In the most detailed study of its kind, the researchers calculated that a 20% tax on sugary drinks - adding roughly 12p to a can of fizzy pop - would reduce purchases by around 15%.

That would reduce the average calorie consumption by 28 calories a week, enough to reduce obesity by between 110,000 and 250,000, according to results published in the British Medical Journal.

Dr Adam Briggs of the British Heart Foundation Health Promotion Research Group at Oxford University, and one of the researchers, said: "Sugar sweetened drinks are known to be bad for health and our research indicates that a 20% tax could result in a meaningful reduction in the number of obese adults in the UK.

"Such a tax is not going to solve obesity by itself, but we have shown it could be an effective public health measure and should be considered alongside other measures to tackle obesity in the UK."

The researchers say the impact of the tax would be greatest on young people. On average people aged 16-29 drink around 300ml of sugary beverages a day.

But one of the researchers, economist Professor Richard Tiffin of the University of Reading, warned that the tax would not reduce obesity by enough to justify the hardship it could cause.

Researchers at Reading found some drinks contained 14 teaspoons of sugar

"Taxing food is a big step, especially when spiralling bills are already making households poorer, and will make very little difference if people are unable or unwilling to make healthier choices elsewhere in their lives," he said.

Professor Sir Stephen O'Rahilly from the Medical Research Council's Metabolic Diseases Unit in Cambridge agreed that sugary drinks are "part of the problem"' but he added that taxation was "politically undeliverable" in most democracies.

He said: "A workable alternative might be to encourage the major companies to switch to the aggressive promotion and marketing of less harmful versions of their products.

Soft drinks firms said there was evidence a tax would not curb obesity

"This could be achieved by balancing a 20% tax on sugared products with a 20% subsidy on artificially sweetened versions of the same beverages."

Any excess calories, whether from protein, carbohydrate or fat, can add on the pounds. Sugar is a carbohydrate.

Gavin Partington of the British Soft Drinks Association said there was "ample" evidence that taxing soft drinks will not curb obesity.

"Trying to blame one set of products is misguided, particularly when they comprise a mere 2% of calories in the average diet."

Mr Partington said: "I challenge the whole basis of this assumption. The truth is we have a problem with obesity because too many people are leading inactive lifestyles and their overall diet is insufficiently balanced. It’s not a case of picking on one product or another, that’s a really cheap gimmick."